In response to the demands of consumers who are driven both by ever-escalating fuel prices and the dire consequences of global warming, the automobile industry is slowly starting to embrace the need for ultra-low emission, high efficiency cars. While some within the industry are attempting to achieve these goals by engineering more efficient internal combustion engines, others are incorporating hybrid or all-electric drive trains into their vehicle line-ups. To meet consumer expectations, however, the automobile industry must not only achieve a greener drive train, but must do so while maintaining reasonable levels of performance, range, reliability, and cost.
In recent years, electric vehicles have proven to be not only environmentally friendly, but also capable of meeting, if not exceeding, consumer desires and expectations regarding performance. While early electric vehicles used DC motors in order to achieve the variable levels of speed and torque required to drive a vehicle, the advent of modern motor control systems utilizing direct torque control have allowed AC motors to deliver the same level of performance while providing the many benefits associated with AC motors including small size, low cost, high reliability and low maintenance.
Although the prior art teaches a variety of direct torque control systems that may be used with an AC motor, these systems typically allow high, potentially harmful transient over currents that can damage the electrical system by causing the power switching semiconductor devices to exceed their operating limits. Transient over currents may occur whenever the flux and torque are rapidly ramped up from zero or during transient operation when the torque command changes rapidly. In addition to transient over currents, large torque ripples may occur during both transient operation and steady-state high torque operation. Torque ripples can shorten a motor system's life and have undesired consequences on the load such as mechanical vibrations and acoustic noise. Accordingly, what is needed is an AC motor control system that prevents the occurrence of high transient currents and limits torque ripples. The present invention provides such a control system.